before deflating the boat (after wiping it), i lay the bag open on the floor and set the boat right over it. With experience you learn which way works best and in which position to place the bag relative to the boat.

Then you deflate the boat, fold it as per instruction and you end up (most of the time) with the folded boat right on the bag. Then you slide the corners into it...

works great. I also us a plastic sheet to lay on the floor when only muddy or very wet surfaces are available.